Macrophytes In Eutrophic Shallow Lakes Research Articles

Functional traits of submerged macrophytes in eutrophic shallow lakes affect their ecological functions

Trait-based approaches have been widely used to explore the relationships between submerged macrophytes and their surrounding environments. However, the effects of functional traits on ecological functions of submerged macrophytes in eutrophic lakes are still not well understood. Here, 1745 individuals of eight dominant submerged macrophyte species in 19 Yangtze floodplain lakes were collected and classified as needle-leaf (Myriophyllum spicatum, Ceratophyllum demersum, Stuckenia pectinatus, Najas minor) or flat-leaf (Vallisneria natans, Hydrilla verticillata, Potamogeton wrightii, Potamogeton maackianus) types according to photosynthetic trait-based cluster analysis. The flat-leaf type submerged macrophytes possessed greater photosynthetic (e.g. higher Fv/Fm) and morphological traits (e.g. higher SLA), while the needle-leaf types held greater stoichiometric traits (e.g. higher plant N/P). Moreover, the RDA analysis indicated that water depth (distribution depth of submerged macrophytes) was the key factor influencing functional traits of flat-leaf types, while it was water quality (e.g. WTP and WChl a) for the needle-leaf types. Furthermore, the flat-leaf types showed better performance in improving underwater light conditions (e.g. SD, Kd, Zeu/WD and Red/Blue) and water quality (e.g. WChl a and TSM). Additionally, distribution depth (WD) of the flat-leaf types was shallower than the needle-leaf types in eutrophic shallow lakes along the middle-lower reaches of the Yangtze River. Our study highlights that functional traits of submerged macrophytes in eutrophic shallow lakes affect their ecological functions.

Mesocosm experiment reveals a strong positive effect of snail presence on macrophyte growth, resulting from control of epiphyton and nuisance filamentous algae: Implications for shallow lake management

Increased nutrient loading has adverse effects on the growth of submerged macrophytes in eutrophic shallow lakes. Where growth of phytoplankton, epiphyton and filamentous algae is excessive, all may contribute to shading that limits macrophyte growth. However, when abundant, herbivorous snails may dampen this effect by reducing the biomass of epiphyton, and perhaps also of nuisance filamentous algae, both which have the potential to become more abundant in a future warmer world. We studied the effects of herbivorous snails (Radix swinhoei) on the biomass of phytoplankton, epiphyton and filamentous algae as well as the growth of the submerged macrophyte, Vallisneria denseserrulata, under contrasting nutrient loadings (low, nitrogen (N) 113 μg L−1·d−1 and phosphorus (P) 10 μg L−1·d−1; high, N 339 μg L−1·d−1 and P 30 μg L−1·d−1) in a 30 day outdoor mesocosm experiment, conducted on the shore of subtropical Lake Taihu, China. We found significant interactive effects of nutrient loading and snail presence on biomasses of epiphyton and filamentous algae and on the biomass and relative growth rate of submerged macrophytes. When snails were absent, the biomass of epiphyton and the biomass and coverage of filamentous algae all increased markedly, while the biomass, density and relative growth rate of V. denseserrulata decreased significantly with increased nutrient loading. When snails were present, biomasses of epiphyton, phytoplankton and filamentous algae were significantly reduced and growth of V. denseserrulata significantly increased under both high and low nutrient loading scenarios, and the effect was most pronounced in the nutrient-rich treatment. The present study suggests that in shallow aquatic ecosystems, herbivorous snails reduce the negative impact of nutrient loading on submerged macrophyte growth, by controlling both epiphyton and nuisance filamentous algae. How best to protect snails from fish predation in order to realize this potential under natural conditions is a matter that warrants further studies.

Omnivorous shrimp Neocaridina denticulata sinensis enhances the growth of submerged macrophyte Vallisneria denseserrulata

Lake eutrophication often causes declines and even losses of submerged macrophytes through the shading effects of increased periphyton and phytoplankton. The Chinese swamp shrimp Neocaridina denticulata sinensis Kemp (Decapoda, Atyidae) is a common omnivore in Chinese lakes, where its presence may impact both periphyton and phytoplankton, with previously unstudied consequences for submerged macrophytes. Here, using a mesocosm experiment, we studied the effect of N. d. sinensis on periphyton, phytoplankton and the submerged macrophyte Vallisneria denseserrulata. Results showed that in the presence of N. d. sinensis, the biomass of periphyton on the leaves of V. denseserrulata was significantly reduced, and that growth rate of V. denseserrulata increased. The presence of N. d. sinensis also significantly increased the total phosphorus concentrations in the water column and phytoplankton biomass (chlorophyll-a). The enhanced growth of V. denseserrulata is likely to be linked to improved light harvesting due to the reduced periphyton attached to their leaf surface. The results suggest that stocking with Chinese swamp shrimps may enhance the development of macrophytes in eutrophic shallow lakes.

A MODIS-Based Novel Method to Distinguish Surface Cyanobacterial Scums and Aquatic Macrophytes in Lake Taihu

Satellite remote sensing can be an effective alternative for mapping cyanobacterial scums and aquatic macrophyte distribution over large areas compared with traditional ship’s site-specific samplings. However, similar optical spectra characteristics between aquatic macrophytes and cyanobacterial scums in red and near infrared (NIR) wavebands create a barrier to their discrimination when they co-occur. We developed a new cyanobacteria and macrophytes index (CMI) based on a blue, a green, and a shortwave infrared band to separate waters with cyanobacterial scums from those dominated by aquatic macrophytes, and a turbid water index (TWI) to avoid interference from high turbid waters typical of shallow lakes. Combining CMI, TWI, and the floating algae index (FAI), we used a novel classification approach to discriminate lake water, cyanobacteria blooms, submerged macrophytes, and emergent/floating macrophytes using MODIS imagery in the large shallow and eutrophic Lake Taihu (China). Thresholds for CMI, TWI, and FAI were determined by statistical analysis for a 2010–2016 MODIS Aqua time series. We validated the accuracy of our approach by in situ reflectance spectra, field investigations and high spatial resolution HJ-CCD data. The overall classification accuracy was 86% in total, and the user’s accuracy was 88%, 79%, 85%, and 93% for submerged macrophytes, emergent/floating macrophytes, cyanobacterial scums and lake water, respectively. The estimated aquatic macrophyte distributions gave consistent results with that based on HJ-CCD data. This new approach allows for the coincident determination of the distributions of cyanobacteria blooms and aquatic macrophytes in eutrophic shallow lakes. We also discuss the utility of the approach with respect to masking clouds, black waters, and atmospheric effects, and its mixed-pixel effects.

In-lake algal bloom removal and submerged vegetation restoration using modified local soils

A “modified local soil induced ecological restoration” (MLS-IER) technology was developed for the restoration of degraded shallow lakes. Modified local soils that mixed with macrophyte seeds were used to flocculate the algal blooms and sink them down to the bottom of the lake. The increased water clarity and the improved sediment quality due to the covering of clean modified local soils make it possible for a quick restoration of submerged macrophytes in eutrophic shallow lakes. The MLS-IER technology was tested in the whole bay of Liaoyangyuan (0.1km2) in Lake Tai (Wuxi, China) in August 2006. The whole bay was fully covered by more than 1cm cyanobacterial bloom since June, which caused massive killing of fish and aquatic vegetations. Some 4tons of chitosan-modified local soils were sprayed over the whole bay and the severe bloom was successfully removed within one day. The secchi depth was increased from 0cm to 30cm, and the chlorophyll-a, total-P, and total-N were all reduced by more than 86% within one day's time. Four months after the treatment, submerged macrophytes were successfully restored within the whole bay. Cyanotoxin microcystins RR and LR were reduced by 50% and 40%, respectively, compared to those outside the bay 4 months later. The biodiversity index of zoobenthos and that of phytoplankton inside the bay became higher than that outside the bay, while zooplankton diversity index remained relatively unchanged. This field trial study indicated that restoration of submerged macrophytes in shallow lakes could be significantly accelerated by using MLS-IER technology. The long-term ecological response and the transition mechanism between algal cells and submerged macrophytes in the sediment need to be further studied in controlled whole lake experiments.

A mosaic community of macrophytes for the ecological remediation of eutrophic shallow lakes

The restoration of macrophytes in eutrophic shallow lakes has been the focus of active research in the past decade. The approach of building up a mosaic community of macrophytes (MCMs) to improve water quality and remediate the ecosystems of lakes is proposed in this paper. Several species of floating, floating-leaved, and submerged macrophytes were introduced in experimental enclosures in eutrophic shallow lakes. These macrophytes were intercropped in small patches and formed mosaic communities of spatial and temporal combinations (spatial and seasonal mosaic patterns) in the lakes. Macrophytes can improve water transparency quickly and the MCM system can stabilize this clear water state over a long time in turbid eutrophic shallow lakes. The constructed MCM created heterogeneous habitats that are favourable for different macrophytes and for the growth and succession of other organisms, as well as for removing water pollutants. When the eutrophic water flowed through the MCM system at a retention time of 7 days, the removal efficiency rates of the MCM system for algae biomass, NH 4 +–N, TN, TP and PO 4 3−–P were 58%, 66%, 60%, 72% and 80%, respectively.